An optical recording medium may be roughly classified into a read-only medium, such as a so-called compact disc, and a medium on which signals may be recorded, such as a magneto-optical disc. With these recording media, it is desired to raise the recording density further because digital video signals as recording signals are in need of a data volume several to ten times or more the data volume for digital audio signals, and also because there is a demand for further reducing the size of a product such as a player by reducing the size of the recording medium such as a disc even when recording the digital audio signals.
Meanwhile, the recording density of the optical recording medium is governed by the line density along the scanning direction of the recording tracks and the track density conforming to an interval between adjacent tracks in a direction perpendicular to the scanning direction. The opto-physical limits of the line and track densities are governed by the wavelength .lambda. of a light source and the numerical aperture NA of an objective lens. In general , a readout limit of 2NA/.lambda. is generally accepted as for a spatial frequency during signal reproduction. For this reason, realizing high density in an optical recording medium, it is thought to be necessary to shorten the wavelength .lambda. of the light, source of the reproducing optical system, such as a semiconductor laser, and to increase the numerical aperture NA of the objective lens.
However, since there is also a limit, in improving the wavelength .lambda. of the light, source or the numerical aperture NA of the objective lens, research is under way for improving the structure of the recording medium and the reading method to increase the recording density.
The density in a direction at right, angles to the laser scanning direction (recording track direction) and the pitch arraying distance (so-called track density and a track pitch as a reciprocal thereof) are hereinafter scrutinized. The soot diameter produced on irradiation of the recording medium with the laser light, (diameter of a so-called first dark ring) is given by EQU 1.22.lambda./NA
where .lambda. is a wavelength of the laser light and NA the numerical aperture of an objective lens. For NA=0.5 and .lambda.=780 nm (0.78 .mu.m), the spot, diameter is 1.9 .mu.m. Although a usual limit, of 1.5 to 1.6 .mu.m is set for a pit, arraying interval in the direction at right, angles to the laser scanning direction (so-called track pitch) because of crosswalk from neighboring tracks, it is desired to improve the limit further.
In view of the above described status of the art, it is an object of the present invention to provide an optical recording medium in which the above-mentioned crosstalk may be diminished by a simplified arrangement, for further improving the so-called track density (recording density in a direction at right angles to the laser light, sweeping direction).